I pull the valves off, pop them apart, and toss them into the keg. I fill it up with water and a couple scoops of Oxyclean. I put the lid on and let it sit for an hour or so. Then I dump half the solution out, and scrub the hell out of it with a toilet brush, with the valves still in the bottom. I rinse it out really well, and put it all back together.
Then I fill it with a sanitizng solution, using either bleach or iodopher. This is what I've done in my limited experience with kegging, but it seems to be easy and effective.